It has been found to be advantageous to collect the hot combustion gases produced in a glass melting furnace or unit and to pass them in heat-exchange relationship with the batch material being supplied to the melting furnace. The batch can thus be preheated to elevated temperatures to save significant amounts of energy subsequently required to melt the batch. The exhaust gases otherwise are simply expelled to the atmosphere in many instances with a considerable waste of heat and energy.
Preferably, the heat-softenable batch material is in the form of balls or pellets in the heat-exchange chamber through which the hot gases are passed. However, it has been discovered that the pellet size must be substantially uniform. Otherwise, pellets of varying sizes tend to nest and provide excessive restriction to the flow of the gases past the pellets in the chamber. It has also been discovered that pellets size is important in addition to uniformity. If the pellets are too small, again undue restriction to the flow of the hot gases results. If the pellets are too large, their surface-to-weight ratio is accordingly reduced and the heat transferred to them is accordingly decreased. Also, trapped moisture in the larger pellets may turn to steam and cause the pellets to explode. Specifically, it has been found that pellets of one-half inch nominal diameter with a range from three-eights inch to five-eights inch in diameter are the ultimate for obtaining maximum heat transfer from the hot exhaust gases to the pellets.
The pellets of the heat-softenable batch material preferably are made in a modified commercially-available pelletizer. The components of the batch are mixed together and then supplied to the pelletizer. During transportion to the pelletizer, the batch components tend to segregate so that the actual batch supplied to the pelletizer will vary, even though the final pellets produced and supplied to the melting furnace or unit average out so that the short variations are not material. However, the short variations in the batch components tend to affect the pellet-forming ability of the batch and the size of the pellets produced, other factors being constant. The feed rate of the batch to the pelletizer will also vary and thereby also affect pellet forming and pellet size. Liquid, and specifically water, is also supplied to the pelletizer with the batch supply. With the batch component or quantity variation, different size pellets will result when the water quantity is held constant. However, it has been found that the water quantity, or the ratio of the batch to the water, will also affect the pellet size, with more water resulting in larger pellets and less water resulting in smaller pellets, at least in most instances.
It has also been discovered that measuring a characteristic of the batch in the pelletizer during the formation of the pellets can result in forecast or prediction of pellet size so that the quantity of water or batch to water ratio can be changed to avoid an undesired increase or decrease in pellet size prior to its happening. For example, the depth of the batch material in the pelletizer at certain portions thereof can be measured and the water flow changed accordingly. An increased depth of the nuclei or seed of the batch material indicates that water content is higher, the water tending to cause the seeds to stick together more and thus build up higher. Consequently, the amount of water supplied to the pelletizer is reduced when the sensing device indicates that the batch depth has reached a predetermined value. The excess water would otherwise tend to make fewer but larger diameter pellets, if not reduced. At the same time, if there is too little water, the depth of the nuclei or seeds of the batch decreases with the amount of water then being increased. The lesser amount of water otherwise would result in the individual final pellets thereby being smaller but in greater quantity.
Among the pertinent prior art is a U.S. Pat. application of Stephen Seng Ser. No. 809,595 filed June 24, 1977, which is assigned to the common assignee with this application. That disclosure substantially describes the foregoing. Additionally, an application of Richard K. Henry, Ser. No. 974,470, which was filed on Dec. 29, 1978, now abandoned, a continuation-in-part application, Ser. No. 095,268, having been filed Nov. 29, 1979, both of which are assigned to the common assignee disclose directly sensing pellet size as a sensed characteristic and proportionally controlling the liquid inflow responsively.
However, regardless of the type of characteristic measured and the type of particulate matter or liquid being used in the pelletizer, both the prior application of Seng and the co-pending application of Henry indicate that control of the proportion of dry and liquid material is necessary in order to control the quality of the pellets being produced in the pelletizer.
The co-pending application of Richard K. Henry further indicates that it is also desirable to control the amount of liquid to the amount of dry material going into the pelletizer. This means that within practical limits, certain pellet compositions require a strict proportion of liquid to dry particulate matter be maintained in the pelletizer and that any variation in either the liquid or dry particulate matter be matched by a proportional compensating adjustment for the dry or the liquid matter respectively going to the pelletizer.
Additional prior art references, are U.S. Pat. Nos. 4,031,175 and 3,277,218 which show proportional control of the liquid in flow. However, they do not show a rotary pelletizer wherein a varying characteristic of the material is sensed and the transduced signal is conditioned to match the variations of the characteristic about a desired level. They further do not show a modulator circuit as necessary to match the input requirements of a large control system while producing a signal matching the variations of the material in the pelletizer.